Annealing of alpha-decay damage in zircon: a Raman spectroscopic study

Abstract

Recrystallization and structural recovery in -decay damage in zircon samples have been studied using Raman spectroscopy. Fifteen zircon samples with different degrees of radiation damage have been thermally annealed between 600 K and 1800 K for up to 28 days and 8 hours. The experimental results from this study reveal that recrystallization in the damaged zircon samples is a multi-stage process that depends on the degree of initial damage of the samples. In partially damaged samples the lattice recovery of damaged crystalline ZrSiO 4 takes place at temperatures as low as about 700 K, as shown by a remarkable band-sharpening and a significant increase in the frequencies of 1 and 3 Si-O stretching vibrations together with the external band near 357 cm -1 with increasing temperature. A dramatic increase of Raman scattering intensity of ZrSiO 4 occurs in partially damaged samples near 1000 K due to a recrystallization process involving epitaxial growth. Heavily damaged samples tend to decompose into ZrO 2 and SiO 2 at high temperatures. Tetragonal ZrO 2 has been observed under annealing between 1125 K and about 1600 K in heavily damaged samples while monoclinic ZrO 2 appears above 1600 K. Weak signals from ZrSiO 4 were detected at 1125 K in highly metamict zircon although the main recrystallization appears to occur near 1500 K accompanied by a decrease of the volumes of ZrO 2 as well as SiO 2 . This suggests that this recrystallization is associated with the reaction of ZrO 2 with SiO 2 to form ZrSiO 4 .